Fourth Study Conference on BALTEX Scala Cinema Gudhjem

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Characteristics of the Atmospheric Boundary Layer over Baltic Sea Ice
Burghard Brümmer, Amélie Kirchgäßner und Gerd Müller
University of Hamburg, Meteorological Institute, Bundesstr. 55, 20146 Hamburg, Germany, bruemmer@dkrz.de
1. Introduction
The annual maximum ice cover of the Baltic Sea varied
between 5.2 and 40.5 ⋅ 10 4 km 2 (for comparison: area of
Germany 35 ⋅ 10 4 km 2 ) in the last 50 years. Beside the
annual variation a large intra-seasonal variation of the ice
cover is caused by more or less frequent changes of cold
and warm weather episodes. The presence of sea ice
strongly influences the air-sea energy exchange and the
vertical structure of the atmospheric boundary layer
structure. During two field campaigns, BASIS (BALTEX
Air Sea Ice <strong>Study</strong>) 1998 and 2001, the atmospheric
boundary layer over sea ice in the Bay of Bothnia was
measured. A wide spectrum of boundary layer characteristics
was encountered with weather conditions ranging
from cold-air advection with -25°C to warm-air advection
with +5°C as they are typical for each winter. This study
uses radiosonde and surface measurements over landfast
ice at two stations (ship and ice camp at about 100 km
distance) during 28 days and measurements along 6000
flight kilometres over landfast ice, drift ice and open
water by two aircraft on 16 different days.
2. Results
The paper presents results with respect to the underlying
ice surface (surface temperature and albedo), the horizontal
inhomogeneity of the heat and moisture fluxes at the
air-ice/water interface, and the vertical structure (temperature
inversions, low-level jet) of the atmospheric
boundary layer.
The albedo of landfast ice varied between 0.35 and 0.90
and correlates well with the air temperature. The albedo
of drift ice had always values between the albedo values
for landfast ice and open water and shows a linear relationship
to the surface temperature of the drift ice.
The turbulent heat fluxes ranged from -100 to 250 Wm -2
and the turbulent moisture flux from -20 to 250 Wm -2 .
The smallest fluxes occurred with strong advection of
warm and moist air from Southwest and the largest fluxes
with strong advection of cold and dry air from Northwest
to Northeast. In case of warm-air advection with TAir >
0°C the heat flux was horizontally homogeneous because
under melting conditions open water, drift ice and landfast
ice have about the same surface temperature. Large
horizontal flux variability was present in case of cold-air
advection and ranged from -10 Wm -2 over landfast ice to
250 Wm -2 over thin drift ice and open water (Fig. 1). The
average heat flux over landfast ice is negative.
The atmospheric boundary layer was capped by a temperature
inversion below 1 km in 96 % of the time. A
surface-based inversion and surface-based isothermal
layer occurred in 59 % and an elevated inversion in 37 %
of the time (Table 1). On the average, the surface-based
inversion extended to 165 m with a temperature increase
of 2.3 K, while the elevated inversion hat its base at 340
m and was 185 m thick with a temperature increase of 2.0
K. A weak diurnal cycle was present with less frequent
surface-based inversions and more frequent elevated
inversions around the noon time.
A low-level jet below 0.8 km was present in 86 % of the
time. On the average it occurred at 243 m height with a
speed of 13.3 ms -1 which was 7.1 ms -1 higher than the 10
m wind speed (Table 2). The low-level jet was situated
most often around the inversion top in case of surfacebased
inversions and around the inversion base in case of
elevated inversions.
The results for the atmospheric boundary layer over
Baltic Sea ice are compared with results of a similar
study for the Arctic sea ice.
Fig. 1: Satellite heat flux H averaged over 4 km measured
by Dornier aircraft on 21 Feb 2001 at 10-15 m
height over various surface types.
1998 2001
A K F A D
No. of Temp. Profiles 68 71 35 27 42
Sfc Inversions (%)
Sfc. Isotherm. Layers (%)
Elevated Inversions (%)
56 51 17
10 14 9
34 35 74
67 40
11 14
19 45
Table 1: Number of temperature profiles measured by
radiosonde (A = RV Aranda, K = Kokkola) and aircraft
(F = Falcon, D = Dornier) and percentage frequency of
surface-based inversions, surface-based isothermal layers
and elevated inversions.
1998 A
K
F
2001 A
D
N Us (ms -1 ) ULLJ (ms -1 ) hLLJ (m)
51
55
19
23
35
7.0
4.0
7.2
6.9
7.5
14.4
13.3
10.0
12.6
14.0
255
279
220
243
183
Total 183 6.2 13.3 243
Table 2: Number (N) and mean characteristics of the
low-level jet LLJ (hLLJ = height of LLJ, U LLJ = wind
speed of LLJ, Us = wind speed in the surface layer
measured by radiosonde (A = RV Aranda, K = Kokkola)
and aircraft (F = Falcon, D = Dornier).